The present invention relates to a process for preparing isocyanates and an apparatus suitable for this purpose, and also its use.
To prepare isocyanates by phosgenation of the corresponding amines, there is in principle a choice between a liquid-phase phosgenation and a gas-phase phosgenation. In the gas-phase phosgenation, the reaction conditions are selected so that at least the reaction components diamine, diisocyanate and phosgene are gaseous under these conditions. The present invention relates exclusively to gas-phase phosgenation.
EP 1 275 639 A1 describes the gas-phase phosgenation of (cyclo)aliphatic diamines in a reaction zone having constrictions of the walls.
EP 1 275 640 A1 describes the gas-phase phosgenation of (cyclo)aliphatic diamines and triamines in a mixing tube with reactor, in which the gas flow is accelerated in the mixing region. A reactor geometry having an inner tube and an outer tube, by means of which a gas-phase phosgenation is possible only on a scale of about 211 g of 1,6-hexamethylenediamine per hour, is disclosed.
A disadvantage of the reactors disclosed in these two documents is that if this reactor geometry were simply to be enlarged to industrially useful sizes, the diameter of the inner tube would have to be increased to such an extent that mixing of the streams introduced through the two tubes would no longer possible in short mixing times owing to the long distances transverse to the flow direction.
EP 1 449 826 A1 discloses a predistribution of the amine-comprising stream over at least two individual amine lines. However, these are hydrodynamically decoupled from one another. If the cross section of one of these tubes is reduced for example by formation of amine hydrochlorides, the throughput through this tube automatically becomes smaller due to the increase in the pressure drop and leads to reduced flow. However, smaller throughputs lead to even more solids depositing on the wall, so that blocking of the tube proceeds even faster.
DE 10359627 A1 discloses a gas-phase phosgenation in which amine is mixed in through a concentric annular gap between two phosgene streams, with the cross-sectional areas of the two phosgene streams having a ratio of from 1:0.5 to 1:4.
This reactor geometry, too has the disadvantage that when this reactor geometry is simply enlarged to industrially useful size, the internal diameter of the mixing device can be increased only up to a particular limit in order for this area ratio to be adhered to.
WO 02/02217 describes various methods of mixing feed streams, including streams for a liquid-phase phosgenation.
A disadvantage is that the methods disclosed there are intended for the mixing of liquid phases at low entry velocities of only about 10 m/s, while the significantly higher velocities and the mixing of gases required in gas-phase phosgenations are subject to different fluid-dynamic requirements than the mixing of liquids. In addition, the reaction rates of a phosgenation in the gas phase are significantly different from those in the liquid phase, so that the method of WO 02/02217 cannot be readily applied to the gas phase.
It was an object of the present invention to develop a method for carrying out a gas-phase phosgenation which can be implemented on an industrial scale.